125 Stimulation of the hypothalamus and pituitary with exogenous kisspeptin and GnRH showed a similar response in LH pulsatile secretion in ovariectomized Bos taurus and Bos indicus beef heifers.

In the U.S., Bos indicus-influenced females represent ~30% of the cow herd. Nonetheless, these females have phenotypic characteristics that impair reproductive efficiency compared to Bos taurus females, including delayed puberty and inefficiencies related to the pharmacological control of ovulatory cycles. The mechanisms underlying these differences have yet to be fully elucidated at the hypothalamic-pituitary level. In this study, it was hypothesized that Bos indicus (Brahman) heifers have a decreased LH secretory response following serial stimulation with kisspeptin and GnRH under high progesterone (P4) negative-feedback conditions compared to Bos taurus (Hereford) heifers. Fourteen post-pubertal heifers (Brahman BR; n=8 and Hereford HF; n=6) were ovariectomized and received two estradiol-17β (E2) implants to create basal E2 concentrations. On d-5, all heifers received two intravaginal P4 devices (1.34 g), which were retained throughout the study. On d0, jugular catheters were placed. Each heifer was submitted to three distinct conditions: Kisspeptin (Kiss; 0.4 µg/Kg BW), GnRH (Gonadorelin; 1 µg/Kg BW), and Control (Saline). Intravenous injections were administered hourly for 8h with blood samples collected every 15 min. Circulating LH concentrations were determined via radioimmunoassay and statistical analysis was performed by one-way ANOVA with Tukey’s HSD. Under control conditions, no differences in endogenous LH pulse frequency (BR: 2.4±0.8 pulses; HF: 1.8±0.7 pulses), LH pulse peak (BR: 1.49±0.2 ng/ml; HF: 1.53±0.34 ng/ml), and pulse amplitude (BR: 1.11±0.19 ng/ml; HF: 1.17±0.28 ng/ml) were observed between BR and HF heifers. Nevertheless, under the same P4 suppressive environment, GnRH administration induced concomitant LH pulses resulting in higher LH pulse frequency compared to control conditions but did not differ between breeds (BR=7±0.7 pulses; HF=6±0.8 pulses). Overall, GnRH administration resulted in detectable LH pulses in 86% of BR and 90% of HF heifers. Similarly, Kiss administration successfully stimulated high frequency of LH pulses compared to heifers under control conditions, potentially acting on the hypothalamic pulse generator, but did not differ between genetic groups (BR=5.9±1.1 pulses; HF=5.2±1.1 pulses). Overall, Kiss administration resulted in detectable LH pulses in 86% of BR and 83% of HF heifers. Unexpectedly, LH pulse peak was not influenced by GnRH administration or Kiss stimulation between BR (GnRH=2.29±0.15 ng/ml; Kiss=1.71±0.09 ng/ml) and HF (GnRH=2.44±0.2 ng/ml; Kiss=1.84±0.15 ng/ml) heifers. Additionally, no differences were detected in LH pulse amplitude between the genetic groups during GnRH (BR=1.51±0.12 ng/ml; HF=1.50±0.16 ng/ml) or Kiss stimulation (BR=1.03±0.04 ng/ml; HF=1.84±0.15 ng/ml). In conclusion, both GnRH and Kiss administration effectively increased LH pulse frequency despite P4 suppression, potentially by acting directly at the pituitary and the hypothalamic GnRH pulse generator, respectively. However, no differences in LH response were observed between genetic groups, thus refuting our hypothesis. These results suggest similar responsiveness to GnRH and Kiss stimuli in ovariectomized Bos indicus and Bos taurus heifers under P4 suppression.